crimson/osd: implement CEPH_OSD_OP_SETALLOCHINT in OpsExecuter
This commits brings support setting allocation hints to `OpsExecuter`.
What is important to note that `SETALLOCHINTS`, at the ops execution
layer, behaves basically like `TOUCH`, and thus should be ignored
(for now) at the object store layer to not miss the part constituted
by `PGBackend::maybe_create_new_object()`.